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Old 25th February 2011, 07:23 PM   #1
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Default estimating power rating of Telefunken transformer

Hi,

I was wondering if there is an easy way to "guestimate" the power rating of an unknown power transformer based on e.g core size and DC resistance of the different windings.
The reason I am asking: I have a couple of power transformers in my scrap box that seem to be salvaged from Telefunken radios. I got the transformers as they are, so I have no idea what tube complement the radios had.
The transformers are labeled "Telefunken Bv 37.5021.078-09 Bv/Pv".
Google did not bring up anything useful. The primary has taps for 110, 165, 198 and 220 V, the two secondary deliver 245V and 6,3V, respectively.
The core is 78mm x 65mm x 33,5mm (sorry for the metric scale - roughly 3 x 2.5 x 1.3 inches if my math is correct).
I was wondering if I can use these for a simple EL84 SE amp (I guess, alltogether this will need an 60 or 70VA rated transformer).

Thanks for any advice.

Martin
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Old 25th February 2011, 08:10 PM   #2
Knarf is offline Knarf  Denmark
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How much does it weigh?

Old Skool consumer grade EI core transformers are rated at about 40-45 VA of continuous load per kg of weight, all included. That is the figure I have gotten from weighing some of my vintage transformers of known specs. Use less if you want to run your vintage components more gently.

To estimate VA load, you just add up the secondary loads of filament and HT current, corrected for power factor.

Per channel it would be about 6.3 * 1.05 A for filament current, assuming one EL84 and something like an ECC83 per channel. That is 6.6VA, as the PF correction is 1.0 for resistive loads, like tube filaments.

For HT you first figure out the full DC current, which is probably around 50mA per channel, maybe a bit higher if you include driver stages and bleeder current. Most of that current is for the output tube, of course. Then triple this figure as a rough estimate to correct for power factor of rectifier with capacitor input filter. Total VA load of HT is then 245 * 0.05 * 3 = 37VA.

So VA load per channel is about 43VA, meaning your transformer should weigh about 1kg per channel you want to use it for.

Note that modern, consumer grade tube transformers, like Hammond, run at about 60VA per kg.
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Old 25th February 2011, 08:17 PM   #3
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Hi Knarf,

thanks for the fast and detailed reply.
I will check the weight of these once I am back home. From memory I would guess they are around 1-1.5 kg, so should be fine.
I might build a temporary setup of the amp first and check how hot the transformer gets before actually drilling holes in the chassis.

Thanks again,
Martin
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Old 25th February 2011, 08:42 PM   #4
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diytube.com :: View topic - The Dumpster Transformer Topic
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Old 25th February 2011, 08:43 PM   #5
Knarf is offline Knarf  Denmark
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Quote:
Originally Posted by bayermar View Post
I might build a temporary setup of the amp first and check how hot the transformer gets before actually drilling holes in the chassis.
Hi Martin,

No need to drill any holes if you have a variable, low voltage PSU on your workbench. There is a quick and dirty way of checking the heating before you lift your soldering iron:

Measure all DC resistances of all primary and secondary windings. Calculate rough power loss in each for the rated load, like this:

Say you estimate VA load at 45VA.

Primary: Resistance 5 ohm. Primary current: 45VA / 230V = 0.196 Arms.
Primary power loss: 5 ohm * 0.196A^2 = 200mW.

Secondary (HT): Resistance: 100 ohm (may be much higher). Power loss: (0.05A * 3.0)^2 * 100 = 2.25W.

Secondary (filament): Resistance: 0.2 ohm. Power loss: 0.2 * 1.05^2 = 220mW.

Total ohmic losses roughly 2.7W. Add 20% for core losses, say 3.2 W.

Important: Connect a 1A or larger diode, like an 1N4007 or similar, directly across the HT secondary. Now apply a DC voltage from your bench PSU, calculated to dissipate 3.2W in the HT secondary. V = sqrt( 3.2W * 100 ohm) = 18V in my example. Connect plus to diode cathode.

Leave power connected for a while, while keeping an eye on the transformer temperature. When temperature has stabilized, decide if the transformer is too hot for your liking, considering age, condition etc.

When you disconnect the DC voltage, make sure the diode stays connected. It is there to protect the transformer insulation and/or your PSU from being damaged by the back EMF voltage, which would be generated on disconnect if the diode wasn't there.

Tweak to your own measurements. Depending on the actual resistances, I often run different primaries and secondaries in series and/or parallel to get a total resistance in a suitable range.

- Frank.
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Old 25th February 2011, 08:51 PM   #6
Knarf is offline Knarf  Denmark
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Quote:
Originally Posted by boywonder View Post
That article has an error. The power factor correction of capacitive filter input is certainly not a third over choke input. It is somewhere in the range between 1.584 (highly unlikely) and up to 6.0 in extreme cases. Factor of around 3.0 is typical for the components used in many tube PSUs, which is why I used that value in my example above.

- Frank.
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Old 26th February 2011, 10:37 AM   #7
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Thanks, Frank (and Boywonder)!
Thanks again for the detailed walk-through.
That procedure makes perfect sense and might put one or the other old transformer back in service.
I only use transformers that still look like almost new ones - I don't want to be cheap where safety is involved.

Martin
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Old 26th February 2011, 10:41 AM   #8
AJT is offline AJT  Philippines
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Quote:
roughly 3 x 2.5 x 1.3 inches if my math is correct)
core area seems to be 1.3 inches squared, therefore va rating of the iron could be 52 volt amperes....
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Old 26th February 2011, 05:04 PM   #9
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Hi Martin,

I think I have the exact same transformers here, but have not put them to use yet (they were intended for a headphone amp). These transformers pop up on german Ebay from time to time, right?

From what was said in the auction they are good for about 80mA at 245V and 3A for the heaters (which is almost dead on what Tony predicted...)

Some of these TFK transformers are a pain in the .... to hook up... took me forever to get it figured out. Also remenber that with a 220V primary your secondaries will be about 260V and ~7V.

regards

PS we are almost in the same neighbourhood...
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Old 28th February 2011, 08:24 AM   #10
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Hi everyone,

thanks for all the input.
This is why I love this forum.

@Stixx: Yes, those transformers might just be similar ones. I think mine came from eBay, too. But either there was no clear description of the power rating or I lost it. The hook-up should be not so difficult in this case since there is a legend for the lead colors printed on the transformer e.g. red-red: 245V etc.
The higher secondary should not be a problem, but thanks for reminding me about the fact.

Thanks again for all the answers.

Martin
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